Schools Minister Jacqui Smith came under fire last week after appearing to give teachers the green light to double class sizes if it is the only way to implement controversial reforms to teachers' working hours.
Under changes coming into force in September, teachers will be entitled to set aside ten per cent of their working week for marking and preparing lessons.
The Professional Association of Teachers' (PAT) annual conference warned that some primary schools lacked the money to hire qualified teachers to cover lessons while other staff took this time off.
Heads are being forced to consider radical options including doubling class sizes and giving unqualified teaching assistants a bigger role, delegates said.
This could mean some classes containing up to 60 pupils with just one qualified teacher in charge, supported by one or two assistants, they said.
The Minister said headteachers should run classes of up to 60 children "if they think that's right for their schools". Speaking to reporters after the conference, she said: "What is important is that headteachers make the decisions that they think are right for their schools in the context of the extra funding we have put into the system." But Ms Smith, MP for Redditch, later attempted to diffuse anger claiming she had not meant to condone bigger class sizes.
"I am in no way advocating classes of 60 pupils," she said.
"A Government which has increased teacher numbers by 32,000 and increased teaching assistants by 86,000 since 1997 is not in the business of increasing class sizes.
"It is in the business of working with the profession to ensure that our children can get even higher quality teaching."